Add an in-language Pkl parser and syntax module

It would be good to have a parser for actual Pkl code, where the parser returns syntax nodes.

This enables use-cases like:

* Linter with in-language defined linting rules
* Source code transformations written in Pkl

The issue names no files, tests, or entry points. Start by locating the repository's existing parsing and syntax components, then clarify the parser and syntax-node scope with maintainers. Done means Pkl source can be parsed into syntax nodes with a tested foundation for the stated linting and source-transformation use cases.

Written by the indexing model from the issue text.